Viewfinders find many uses and are of particular importance in cameras to aid the photographer in framing the subject. In the past, very inexpensive viewfinders have not provided an accurate means to indicate to the photographer the aim of the camera. And the most accurate and capable viewfinders, found in $70-300 point-and-shoot cameras, are too expensive for current $9-$15 single-use cameras. In the very least expensive cameras of the single-use camera type, an optical viewfinder is dispensed with, providing only a hole through the camera which provides virtually no accuracy in aiming the camera. In order to provide accurate framing, viewfinders have been used which require the use of a minimum of two lenses, to which reflective optical coatings are added (the Albada viewfinder).
A number of viewfinders have been devised. The following table describes various viewfinders used in simple cameras, including point-and-shoot, and single-use cameras:
Summary of existing low cost viewfinders:
______________________________________ FORMS IMAGE OF TYPE FRAME COMMENTS ______________________________________ Large hole No Pros: costs nothing. through camera Cons: Frame line out of focus. Not accurate. Acceptable for only the least expensive cameras. Small hole in No Comments: Small hole near eye acts as rear wall of a pin-hole lens. camera. Large Pros: Image of scene and front frame hole through sharp. No motion parallax between front. FIG. 1 frame and scene. Cons: Not usable by eye-glasses wearers. Image is dim. Has limited field view. Reverse- No Pros: Relatively inexpensive. Galilean Cons: Adds cost of lenses and Viewfinder, assembly. Lenses collect dirt, finger FIGS. 2 and 2A prints and scratches. Image is minified making it harder to see Edges of frame are out of focus. Eye motion causes parallax errors. Albada Yes Comments: Created by adding Viewfinder, reflective coatings to reverse FIGS. 3 and 3A Galilean viewfinder lenses. Pros: Bright rectangular frame is imaged in the scene. No motion parallax between frame and scene. Cons: Costs of lenses and assembly. Lenses collect dirt, finger prints and scratches. Image is minified making it harder to see. ______________________________________
The reverse-Galilean viewfinder, FIG. 2, so named because it makes use of one positive and one negative lens, as Galileo used in the first telescope. However, in reverse-Galilean viewfinders, the order of the lenses is reversed which minifies the image.
The Albada viewfinder as illustrated in FIG. 3 uses the same lenses of the reverse Galilean viewfinder, but adds a reflective coating to the concave surface of the negative lens to act as a magnifying mirror. Also a frame or reticle is formed on the interior surface of the positive lens, usually by metal deposition. Two superimposed images are formed, (1) the lenses refract transmitted light to create a minified image of the scene, and (2) some light reflects off of the reticle forward to the concave surface of the front lens, where it is reflected (and focused) rearward to the eye. The images of the scene, and of the reticle, appear to be superimposed at a comfortable viewing distance of about 10 feet.
In each of these types of prior art cameras which are represented in the accompanying drawings, FIGS. 1-3, the field of view of the viewfinder is represented in the vertical dimension as the space between the lines FOV(vf) and typical images seen therein are represented by FIGS. 1A, 2A and 3A, respectively.
The field of view of the user's eye is, of course, larger than that of the camera viewfinder and is represented in these same figures in the vertical plane by the space between the lines FOV(eye).
Many cameras, including low cost point and shoot cameras provide zoom capability. For such applications, a reticle is even more important to accurately frame the photograph.
In the photo finishing business, a cropping aid is used which consists of a pair of planar right angled notched sheets which are movable to define a variable size exposure rectangle. These two sheets, determine the limits of the scene and crop the final reproduction. In certain complex lens type viewfinders similar planar masking devices have been used. This type of adjustment has not, heretofore, been applied to a lensless viewfinder.